The
Placenta & Twinning

Professor A. Cuschieri

Department of
Anatomy

University of Malta

oDistinguish between the foetal and maternal contributions to the
placenta

oList the main functions of the placenta

oDistinguish between identical and fraternal twins and how they arise

oExplain how identical twins can share a common amnion and chorion

Introduction

When the
blastocyst embeds itself in the maternal endometrium it is covered with
chorionic villi derived from the syncitiotrophoblast and cytotrophoblast. The
distribution and size of the villi are not uniform throughout the surface of
the chorion.

The chorion frondosum
consists of numerous villi over the embryonic pole. This will contribute to the
formation of the placenta .

The chorion laeve contains very
sparse villi over the abembryonic pole. The villi will eventually disappear,
and here the chorionic membranes are formed.

The
Placenta is derived from two sources:

• The foetal chorion

• The maternal deciduas

By the third week tertiary chorionic villi are formed.

New chorionic villi continue to
sprout out at different gestational ages

At 9 -16 weeks they are termed mesenchymal villi

At 16 -25 weeks they are
immature intermediate villi.At this
age the cytotrophoblast persist only in patches.

The Placenta
consists of branched chorionic villi bathed in lacunae of maternal blood.

On the foetal surface of the
placenta the chorion forms a continuous surface from which the villi
arise.The cytotrophoblast proliferates
from the tips of the villi and forms a cytotrophoblastic shell on the maternal
surface of the placenta.Elsewhere the
villi are lined by syncitiotrophoblast, while the cytotrophoblast becomes
restricted to small patches.

The Decidua

The decidua is derived from the
secretory endometrium, which continues to proliferate and secrete under the
influence of persistent high levels of progesterone, which in turn is
stimulated by increasing levels of HCG, secreted by the growing
syncitiotrophoblast.As the decidual
cells continue to proliferate, they
accumulate lipids and glycogen, and, the whole decidua becomes more vascular.

Septa grow from the decidua and
project into the intervillous spaces dividing the placenta into 15 to 20 cotyledons.

d.Passage
of cells into the maternal circulation. This is acquiring increasing importance
because new methods are being developed to isolate the foetal cells from the
maternal blood, and use them for prenatal diagnosis instead ofthe invasive methods of amniocentesis and
chorionic villus biopsy.

Twinning

There are two types of twins:

a.Fraternal twins are the result of fertilization of two oocytes.The two zygotes develop and implant
themselves separately.They have
separate placentae and amniotic cavities.Like all sibs born to the same couple, they have half their genes in
common.They may be of different sex or
of the same sex.

b.Identical twins are derived from a single zygote, which during early
development divides into two groups of cells that continue to develop
independently.Identical twins are
always of the same sex, and have identical genes.Identical twins are in fact clones as the two individuals are
derived from the same cell.

Identical twins mayshare the
same placenta or amniotic cavity, or they may have different placentae and
amniotic cavities, depending on the developmental stage at which the separation
of the conceptus into two twins occurs:

1. Separation of theblastomeres

into two groups which develop and
implant separately.They result in

two separate placentae and gestational sacs (dichorionic,
diamniotic)

2. Separation of the inner cell mass or embryoblast into two groups
forms two amniotic cavities but one chorion andplacenta (form monochorionic,diamniotic twins)

3. Separation of the bilaminar disc into
two groups of pluripotent cells forms two embryos sharing a single amniotic
cavity, chorion and placenta (form monamniotic, monochorionic twins)

4.Incomplete separation of the inner cell mass gives rise

to conjoined twins.

5.Anastomosis between the circulations of monoamniotic twins may cause failure
of normal growth and development in one embryo